The present invention relates to a multi-phase PSK (Phase Shift Keying) demodulator and, more particularly, to a demodulator of the type including a sweep controller for broadening a synchronization capture range and which is capable of removing false synchronization.
A demodulator of the type described is basically made up of a quadrature detector for detecting an input PSK signal by a recovered carrier wave. Analog-to-digital (A/D) converters are connected to the outputs of the quadrature detector. An error correction/synchronization decision circuit is connected to the output of the A/D converters for producing both demodulated data and a synchronization decision signal. A carrier recovery circuit produces the recovered carrier wave from the outputs of the quadrature detector or the outputs of the A/D converters. A sweep controller controls the frequency of the recovered carrier wave in response to the synchronization decision signal to change the synchronization capture range.
A problem with the above-described demodulator is that when a certain relationship holds between the synchronization capture range and the symbol rate, the demodulator falsely decides that synchronization has been established when it has not been set up. This kind of occurrence is generally referred to as false synchronization and will be described in detail later.